Generally, the invention relates to solutions for coating or impregnating substrates. In particular, the invention relates to chemical solutions which increase the conductivity of a substrate when used as either an impregnating solution or a surface coating. The dimensional stability of porous and permeable substrates (such as cellophane) is improved when impregnated with the solution.
Some types of dielectric copying and printing require a printing medium substrate which is through-conductive, and which contains a dielectric coating on one surface. Usually, the substrate is comprised of a through-conductive material having a top surface and a bottom surface. The substrate is through-conductive when an electric current can be made to flow between the top surface and the bottom surface in response to an electrical potential applied between the surfaces. Substrates such as paper, vellum and cellophane are not normally sufficiently through-conductive. However, by impregnating with a conductive chemical solution, the paper, vellum or cellophane will absorb this chemical solution and become satisfactorily conductive. Previously, chemical solutions such as quaternary ammonium compounds and other polymers have been used as impregnating solutions.
In addition, a method of imparting volume conductivity into non-porous films is to knead the conductive chemical solution into the film compound before casting. This method is particularly appropriate for alcohol soluble or compatable resin films.
Surface conductivity also is a desired characteristic of printing, copying and other substrates. Non-porous substrate materials such as polyester film are not capable of being impregnated by a conductive solution. However, solutions to increase the surface-conductivity can be coated on back and/or front surfaces of the substrate.
In the past, nitrocellulose was used to bind conductive substances to a substrate surface. Conductive pigments such as carbon black or zinc oxide were added to the nitrocellulose to provide a conductive coating. However, the pigments increased the opacity of the coating. Consequently, these coatings cannot be used on x-ray or other films requiring a high degree of clarity.
Cellophane normally holds about 7% water. In spite of a surface coating of nitrocellulose (with or without conductive pigments) being applied to the cellophane back and a dielectric coating on the surface an increase in humidity of the surrounding environment from 70% to 90% relative humidity, produces an increase in water in the cellophane from 7% to 20%. This increase in water content causes an increase in dimensions of the cellophane. Consequently, the surface coating of dielectric face coatings and nitrocellulose back coatings usually crack above 70% relative humidity. Use of other substances for moisture barrier surface coatings on cellophane such as Saran.TM. act as electrical insulators and therefore cannot be used. Also, paraffin wax has been mixed with the nitrocellulose to increase its moisture barrier qualities. However, this wax forms a monomolecular electrical insulating surface on the cellophane and therefore can't be used as conductive coating.
What is desired is a solution which increases the conductivity and the dimensional stability of the cellophane substrate. The substances favored for their conductive and dimensional stability characteristics have not been compatible with one another. For example, the conductive impregnating polymers such as the quaternary ammonium compounds were normally dissolved in polar substances such as water. Calgon 261LV is a water soluble quaternary ammonium compound packaged as 40% polymer dissolved in 60% water. More recently, impregnating solutions utilizing conductive polymers have included ethylene glycol monomethyl ether (EM) or, alternatively, methyl alcohol. While nitrocellulose is totally incompatible with systems containing water, it has been known that nitrocellulose is dissolvable in EM. However, no formulations in the past have been able to combine the extremely good conductivity characteristics of the quaternary ammonium polymers with the extraordinarily good dimensional stabilizing and film forming characteristics of nitrocellulose.
What also is desired are clear, transparent conductive coatings with non-tacky and durable film characteristics that can be coated with good adhesion on both porous and non-porous materials. These conductive coatings are particularly useful in x-ray imaging systems, dielectric copying systems, and anti-static reduction utilizing polyesters and other films.